1. Field of the Invention
The present invention relates to rotary pumps, preferably rotary axial pumps with hydrodynamic bearing, for impelling a liquid through at least one stage with minimum friction and minimum or no shear forces transmitted to the fluid, and more preferably the invention relates to a hydraulic bearing and a continuous axial-flow rotary pump for use in pumping fluids having particles or components the integrity of which must be protected, such as for blood circulation assistance, either in intravascular or extravascular circuits, with no, or at least extremely minimized, damage in the red cells and platelets, and no, or extremely minimized, thrombus formation.
While particular reference will be made in the present specification to a blood pump, it should be understood that the present pump is for use in any other field wherein any fluid must be transferred or conveyed from one place to another one, either in a closed circulation system or in any open circuit or path, the fluid being preferably a fluid where care of its integrity must be taken.
2. Description of the Prior Art
It is well known to provide an axial-flow rotary pump comprising a generically cylindrical casing and/or stator with a rotor, or a plurality of rotors mounted inside the stator to drive a fluid through the pump. The driving of the liquid to transfer the same from an inlet of the pump to a pump outlet is based in the provision of energy to the liquid to increase the fluid pressure thereof. This energy, however, provides several undesired side effects. The elimination of these effects without impairing the pumping efficiency of the pump has been the aim of many developments in the field of pumps, particularly when handling of sensitive fluids, such as explosives, blood, etc., is involved.
Contours, sizes, assemblies and relative positions of the different parts, as well as the stationary and movable surfaces of a pump are aspects and parameters that must be defined when designing the pump. The final objective of the design is to get a maximum efficiency of the pump with a minimum or no side effects resulting from the energy transferred to the fluid during the impelling thereof. Particularly in the case of a blood pump design, the aim is to reach to a pump having a maximum efficiency without side effects causing blood damage and/or blood clotting during operation. Another important objective is to have a pump having a minimum size.
The side effects resulting from the energy transferred during rotation of the pump comprise the generation of secondary or side flows, vortex, cavitation and separation of the flow from the surfaces of the stationary and movable parts of the pump.
The continuous fluid flow behavior through a rotary pump provided with blades is mathematically defined by the Euler equation. According to Euler, pressure energy imparted by the rotor is proportional to the increment of the tangential component of velocity. Analysis of the Euler equation is made through the so called velocity triangles shown in FIG. 1 for a conventional scheme. Vectors represent averaged velocities on a flow surface and the letter references used in FIG. 1 are:
index 1 is used for the pump inlet
index 2 is used for the pump outlet
The Euler equation applied to a conventional rotary pump is:                     (                  R          ·                      C            u                          )            2        -                  (                  R          ·                      C            u                          )            1        =            g      ·      H              η      ·      ω      
where,
H Head
G Acceleration due to gravity
xcex7 Efficiency
if Cu1=0, we have       C    u2    =            g      ·      H                      R        2            ·      η      ·      ω      
This is the reason why traditional pump designs include stator blades at the pump outlet, thus trying to reduce as much as possible the tangential component of the velocity and transform the kinetic energy into pressure energy.
Although many efforts have been made to eliminate or at least reduce the above mentioned side effects, by reducing or eliminating the above tangential component, for example, no solutions have been found hereinbefore. When a small Reynold""s number is involved, that is when one handles small pumps and/or viscous liquids, stator blades at the pump outlet can not effectively reduce the tangential component of the velocity and transform kinetic energy into pressure energy, no matter the shape or number of blades provided. Therefore, flow separation and side flows are formed at the stator blades which cause hemolysis and blood clotting.
It is also well known to provide an axial-flow rotary pump comprising a generically cylindrical casing or stator with a rotor, or a plurality of rotors mounted inside the stator to drive a fluid, such as a liquid, through the pump. The driving of the liquid to transfer the same from an inlet of the pump to a pump outlet is based in the provision of kinetic energy to the liquid to increase the pressure thereof. This kinetic energy, however, while providing the impelling of the fluid it also provides several undesired side effects. The elimination of these effects without impairing the pumping efficiency of the pump has been the aim of the many developments in the field of pumps, particularly when the handling of sensitive fluids, such as explosives, blood, etc., is involved.
Regarding blood pumps, it is known that the rotary pumps for pumping blood, particularly those to be implanted in the human body, for circulatory assistance, causes severe damages in the blood, i.e. hemolysis. The higher or lesser extent at which the blood is damaged will depend on many factors, one the main factors being the high shear forces or stresses affecting the red cells and platelets, such stresses appearing in zones between pump components with relative movements and close to each other or, worst, in contact with each other.
According to Publication No. 85-2185; 1985; from the National Institute of Health (NIH), entitled xe2x80x9cGuidelines for Blood-Material Interactionsxe2x80x9d, it is generally accepted that the quantity of red cells and platelets damaged by shear stresses depends on the intensity or magnitude of the stresses and the period of time the red cell and/or platelet is exposed to the stresses for a determined quantity of hematocrit. The hematocrit is the volume percentage of red cells in the blood. FIG. 3 shows experimental results of blood damage, illustrated in curves corresponding to the tolerance of blood to shear forces, with the shear stresses shown in the Y-axis and the exposure time shown in the X-axis. The region above the curves corresponds to a significant particle destruction. It is shown that the shear stress that can be tolerated by the red cells is below 10 dynes/cm2. There are some regions in the rotary blood pumps, such as in the hydrodynamic bearing housings and in the gap or clearance between the peripheral edge of the pump blades and the inner surface of the stationary casing, housing or stator, wherein the shear forces and stresses generated by the relative movement between the rotor and the casing surfaces exceed the above mentioned tolerated stress value.
The hydrodynamic bearings have shown a good behavior to support mechanical components in relative movement because of the fluid pressure increase in the bearing cavity. This effect requires an important circulating flow to guarantee a continuous operation of the pump and high shear stresses are involved due to the relative speed of the pump components. In gap between the periphery of the blades and the inner surface of the casing a high pressure drop is generated because the high pressure side of the blade and the low pressure side of the blade are joined at this periphery. In addition, like in the hydrodynamic bearings, the shear stresses are high due to the flow speed gradients in the area.
Blood is a tissue composed of plasma and several types of suspended particles having different densities. The plasma is the liquid portion of the blood and is constituted by about 90% of water. While the plasma is not affected, or affected in a lesser extent, by the above mentioned shear forces the particles such as the red cells may be destroyed by such forces and stresses.
Although many efforts have been made to solve or at least reduce the above mentioned problems of the rotary pumps, particularly rotary blood pumps, there is still a need for a blood rotary pump with means for reducing or eliminating the prejudicial shear forces and stresses particularly appearing in the clearances between the rotor and stator or casing, which shear forces are the cause of important damages in the blood integrity.
The following patents describe attempts made to solve the above mentioned drawbacks associated with rotary pumps, more particularly with rotary blood pumps.
U.S. Pat. No. 4,908,012 to John C. Moise, discloses an implantable ventricular assistance pump having tube in which a pump rotor and stator are coaxially contained, and purge fluid is introduced into stator blades of the pump to avoid creation of discontinuities in the blood path wall. The object of this cited patent is reduce the size of the implant and minimize the risk of infection by reducing vibration, minimizing the percutaneous conduit, and directing most of the heat generated by the pump into the blood. No mention to the problem of shear stresses are found neither solved by the patent. Also, the problem of the flow kinetic energy is not addressed and, in fact, the provision of the bladed stator does not reduce the tangential component of the flow speed.
U.S. Pat. No. 5,209,650 to Guy B. Lemieux, discloses a pump integral with an electric motor and impeller assembly that rotates within a stator casing and is supported on hydrostatic radial and thrust bearings so as to avoid having to provide external seals or friction type bearings. As it is clearly disclosed in its specification, the invention addresses the problems that occur with leaking mechanical seals and worn bearings. The problem of shear forces and stresses is not addressed. While Lemieux specifically includes stay vanes pitched to diffuse the liquid from the second stage integral rotor and impeller assembly, the problem of kinetic energy and tangential components of the blood flow is not considered, and it can not be overcome in any way by providing, as disclosed and illustrated in this patent, axial rotors separated by axial stators.
U.S. Pat. No. 5,678,306 to Richard J. Bozeman discloses a method for reducing the damages to the blood by optimizing each of a plurality of blood pump configuration parameters in the known pump components and variations. The process comprising selecting a plurality of pumps components believed to affect blood damage, such as the clearance between the blades and housing, number of impeller blades, rounded or flat blade edges, variations in entrance angles of blades, impeller length and the like. Construction variations are selected for each of the components and these variations are listed in a matrix for comparison of results. Each variation is tested and the total blood damage is determined for the blood pump and, finally, the least hemolytic variation for each pump component is selected as an optimized component. While considerations are made related to the blood damage and the clearance between the housing and the blades, the problem is attempted to be solved by modifying the clearance size and blade-housing geometry without providing any means to seal the clearance at the peripheral edge of the blades.
U.S. Pat. No. 5,055,005 to Kletschka, discloses a fluid pump with an electromagnetically driven rotary impeller levitated by localized opposed fluid forces which levitation eliminates the need for bearings and seals in the driving mechanism. The shear stresses appearing at the levitating areas are dramatically high which causes the blood to be damaged. No considerations are made in connection to means for preventing the blood from damaging under these circumstances.
U.S. Pat. No. 4,382,199 to Issacson discloses a hydrodynamic bearing for a motor driving a pump for an artificial heart. The motor stator has a bore and a rotor with its impeller is slidable and rotatable in the bore. Both rotor and impeller are supported hydrodynamically such that the tendency is for the entire rotor/impeller assembly to be completely suspended by fluid. It is well apparent that high shear stresses will appear between the rotor assembly and the motor stator without effective means being provided to solve this problem.
U.S. Pat. No. 5,049,134 to Golding et al, discloses a blood pump with two hydrodynamic bearings located at the ends of the rotating impeller. The bearings include helical screws for urging the blood through the pump with lubricating and cooling purposes. In addition, the rotatable impeller includes a bore permitting a continuous blood flow from the blades towards the hydrodynamic bearings. The shear stresses in the hydrodynamic bearings are high enough to damage the blood and no solution is provided to this problem.
Other references, such as U.S. Pat. Nos. 3.083.893 to Dean; 3.276.382 to Richter; 2.470.794 to Snyder and 1.071.042 to Fuller provide two or more rotor pumps but they do not address the problem of handling blood and, sealing the gap between the rotors and the casing.
It would be therefore convenient to have a rotary pump, preferably a rotary blood pump, having a minimum quantity of components and capable of providing a continuous flow with minimized, or without, stresses, particularly shear stresses or forces that would damage the circulating fluid, affecting the fluid integrity, particularly blood in a rotary blood pump.
It is therefore one object of the present invention to provide a rotary pump for impelling a fluid, preferably a fluid that must be preserved of any damage, more preferably blood, wherein the rotary pump comprises at least one rotor, a housing and means for forming a seal and/or a bearing at a clearance or gap between the rotor and the housing.
It is a further object of the invention to provide a blood pump with sealing means comprising the provision of a by-passed portion of blood to the clearance between the rotor and the housing, the portion of blood consisting mostly of plasma without red cells, thus preserving the solid particles and red cells from damage.
It is still another object of the present invention to provide a rotary blood pump comprising at least one rotor, a housing or casing and means for by-passing a portion of the blood under pumping into a clearance between the rotor and the casing with the purpose of forming a seal and/or bearing, the portion of blood being taken at a location of the pump where the blood, as a result of centrifugal forces appearing in the blood mass due to the rotation of the rotor, is composed mostly of plasma and other particles, practically without red cells. Therefore, the by-passed portion of the blood, used for sealing and/or bearing purposes, has no red cells which, otherwise would be affected by the shear forces appearing at the clearance between the rotor and housing.
It is also a further object of the present invention to provide a hydrodynamic sealing means for a rotary pump, the pump being of the type comprising at least one rotor arranged within a stationary casing, the rotor comprising a hub and at least one fluid impelling blade in the hub, a gap being defined between a periphery of the rotor and the casing, the sealing means comprising at least one conduit in the rotor for conducting a by-passed portion of the fluid under pumping, the conduit having an outlet located at the periphery of the rotor and an inlet located radially inwardly relative to the outlet, wherein the by-passed portion of fluid enters the inlet of the conduit and exits the outlet of the conduit into the gap to form a pressurized fluid seal between the rotor and the casing.
It is even another object of the present invention to provide a rotary pump for driving fluid, preferably a blood pump, the pump comprising a stationary casing, at least one rotor rotatably mounted in the casing, the rotor comprising a hub and at least one impelling blade in the hub, for impelling the fluid, a gap between a periphery of the rotor and the stationary casing, and at least one conduit in the rotor for conducting a by-passed portion of the fluid under pumping, the conduit having an outlet located at the periphery of the rotor and an inlet located radially inwardly relative to the outlet, wherein the by-passed portion of fluid enters the inlet of the conduit and exits the outlet of the conduit into the gap to form a pressurized fluid seal between the rotor and the casing.
It is a further object of the present invention to provide a continuous axial-flow pump for impelling a fluid under a continuous pattern without side effects to minimize and eliminate damage to fluid, the pump having at least one stage, comprising an outer casing and rotor means mounted in the casing, the rotor means comprising at least two adjacent rotors rotating in opposite directions.